1. Field of the Invention
The present invention relates to a high energy, non-aqueous portable power source such as a high energy density battery or high power electrochemical capacitor, and more particularly, to a high energy, non-aqueous portable power source comprising novel asymmetric alkyl carbonate-based electrolytes which afford improved cycle life, high capacity, and superior low temperature performance.
2. Description of the Prior Art
Non-aqueous electrolytes formulated specifically for application in electrochemical devices optimally possess a variety of desirable qualities, as described in a recent review article by Gores, et al. (1). The authors cite low temperature performance and electrochemical stability as two key requirements for both non-aqueous batteries and capacitors, but do not teach how these requirements may be achieved.
U.S. Pat. No. 5,525,443 (2) discloses a non-aqueous rechargeable lithium ion battery comprising a complex oxide containing lithium, a carbonaceous anode material, and a mixed solvent electrolyte consisting of a lithium salt and a binary blend of a cyclic and acyclic organic carbonate solvent. This patent teaches that in order to maintain electrolyte liquid at -20.degree. C., a mixing ratio of acyclic to cyclic organic carbonate must be in the range of 1 to 4. Solvent combinations outside this ratio form solids at -20.degree. C. which is deleterious to both the capacity and cycle life of the battery.
U.S. Pat. No. 5,474,862 (3) claims the advantage of low temperature performance in lithium ion batteries employing non-aqueous electrolytes comprising a tertiary blend of a cyclic organic carbonate solvent, an acyclic organic carbonate solvent, and an acyclic ester solvent represented by the formula RCO.sub.2 R'. The tertiary combination of organic solvents to give battery electrolytes that perform well at low temperature appears to have been determined empirically. Japanese Kokai application No. JP 08115742 A2 (4) discloses low temperature electrolytes for lithium ion batteries are claimed. These electrolytes consist of a lithium salt, an acyclic carbonate, and at least 10-100 vol % of chloroethylene carbonate.